SPHERE Overview

Introduction

SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) is an extreme adaptive optics system and coronagraphic facility feeding three science instruments: IRDIS, IFS, and ZIMPOL. The primary science goal of SPHERE is imaging, low-resolution spectroscopic, and polarimetric characterization of extra-solar planetary systems. The instrument design is optimized to provide the highest image quality and contrast performance in a narrow field of view around bright targets that are observed in the visible or near infrared. SPHERE is installed at the UT3 Nasmyth focus of the VLT and includes the following sub-systems:

CPI: Adaptive Optics Main Specifications

The common path and infrastructure receives direct light from the telescope, and provides highly stabilized, AO-corrected, and coronagraphic beams to the three sub-instruments. The most important feasibility check for SPHERE observations concerns the AO performance, measured by the Strehl ratio (SR). The SR directly impacts raw contrast level.

AO performance

H-band Strehl Ratio

R-band Strehl Ratio

Good

> 75%

> 20%

Median

50 - 75%

5 - 20%

Poor

< 50%

< 5%

In the following table, we define three regimes with good (H-band SR > 75%, R-band SR > 20%), median (H-band SR = 50-75%, R-band SR = 5-20%) and poor (H-band SR < 50%, R-band SR < 5%) AO corrections as a function of R magnitude and seeing. Note that these values are very conservative, and the system will likely deliver better performance, although this is not yet guaranteed (see User Manual for further details).

NGS mag / Seeing

Seeing < 0.8"

Seeing 0.8" - 1.2"

Seeing > 1.2"

R = 0 - 9 mag

Good

Median

Poor

R = 9 - 11 mag

Median

Median

Poor

R > 11 mag

Poor

Poor

Poor

IFS: Main Specifications

The integral field spectrograph provides a data cube of 38 monochromatic images either at spectral resolution of R~50 between 0.95 - 1.35µm (Y-J) or at R~30 between 0.95 - 1.65µm (Y-H).

Instrument combinations

Dichroics allow IFS and IRDIS to take data in parallel. One dichroic setting allows IFS observations in the Y-H range, and IRDIS observations in K at somewhat reduced performance. Another allows IFS observations in Y-J range, and IRDIS observations in the H band with uncompromised performance.
For IRDIS-alone observations, a mirror is used ensuring optimal K band performance and access to the complete 0.95 - 2.32 µm spectral range.
ZIMPOL can only be used alone.

Offered Modes

For P97, observations using the IRDIS_LSS mode should be requested in Visitor Mode. However, if very good conditions are critical for the observations, this mode can now also be requested in Service Mode under certain conditions explained in the Late Breaking News.

Polarimetric standard stars are not yet part of the calibration plan and must therefore be provided by the user, if needed.

News for P100:

The SPHERE IRDIS DPI mode can be requested in service mode regarless of the filters but users are invited to provide the position angles which minimize the cross-talk effect, maximize the instrument polarization efficiency, see here.

7-hole Sparse Aperture Masking (SAM) is offered as a new setup for the modes: IRDIS_CI, IRDIS_DBI, IRDIFS, IRDIFS_EXT, ZIMPOL_I (S+V) and IRDIS_DPI (V).

Targets with R>11 are now allowed in Service Mode. Since they require the best possible conditions, users should request them with a seeing constraint <= 1.0". For R> 11, H>9 targets, coronagraphs should be avoided as SPHERE might be operated without precise drift control on the focal-plane mask (DTTS-free mode).

*) In general, observations using the IRDIS_LSS or IRDIS_DPI in Pupil tracking should be requested in Visitor Mode. However, if very good conditions are critical for the observations, this mode can now also be requested in Service Mode. A clear justification regarding the need for Service Mode should be provided and will be assessed as part of the technical feasibility review.